Medical treatments for disorders of the nervous system, such as seizure disorders (e.g., epilepsy), have improved in recent decades. One available treatment involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including a reduction of seizure occurrence and the improvement of other medical conditions. An example of such a treatment regimen involves the application of electrical stimulation to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254, which is incorporated herein by reference.
Electrical stimulation of a target tissue of a patient's body (e.g., vagus nerve stimulation) may be provided by implanting an electrical device (known as an implantable medical device, or IMD) underneath the skin of a patient and electrically stimulating the target tissue. Vagus nerve stimulators, cardiac pacemakers, and cardioverter defibrillators are exemplary IMDs. Most IMDs are powered by a battery housed within the IMD. Both rechargeable and non-rechargeable batteries have been used in IMDs. When a non-rechargeable battery is used, the IMD must be surgically removed from a patient's body before the battery is completely exhausted so that a new device (or battery) may be installed. Unfortunately, accurate prediction of battery life can be difficult when the battery is discharged at an uncontrolled rate, e.g., when therapy is delivered on an “as needed” or patient controlled basis, and consequently IMD replacement scheduling is subject to error. Moreover, surgery is costly and inconvenient, and not without risk to the patient. Therefore, it is desirable to avoid or postpone surgery by providing an IMD with longer operational life. The operational life of an IMD may be extended by providing a rechargeable rather than a non-rechargeable battery in the IMD.
Furthermore, when using a non-rechargeable battery, the features and functionalities provided by an IMD are often limited to extend battery life. In order to provide an acceptable operational life without unduly increasing battery size (and consequently increasing IMD size), the functionalities provided by the IMD may be minimized. The value of additional therapy or analysis is considered in light of the impact of the additional features on battery life. Consequently, in IMDs containing a non-rechargeable battery, battery life considerations may preclude providing additional therapy (e.g., more frequent electrical stimulation of tissue) or more computationally intensive analysis of a patient's condition that could ultimately benefit the patient. Rechargeable batteries allow for an increase in IMD energy use without a corresponding decrease IMD operational life, thereby enabling inclusion of features that may not be acceptable in an IMD powered by a non-rechargeable battery.
When using a rechargeable battery in an IMD, a battery recharging system is required. One system for recharging a battery in an IMD involves transcutaneous energy transmission. Transcutaneous energy transmission entails generation of a magnetic field external to the patient's body which induces current flow in a charging circuit of the implanted IMD. The IMD uses the induced current to charge the rechargeable battery.
Unfortunately, transcutaneous energy transmission is not without issues. The efficiency of transcutaneous energy transmission is affected by a number of physical variables. For example, charging efficiency is detrimentally affected if the IMD is not properly aligned with the external charger, or if the distance between the IMD and the external charger is too great. Furthermore, the magnetic field may induce current flow not only in the charging circuit of the IMD, but also in the metallic housing of the IMD. Current flow in the IMD housing is dissipated as heat. If the temperature of the housing becomes too high, the tissue surrounding the IMD may be damaged.
For these reasons, systems and methods for improving the efficiency of transcutaneous energy transmission to an IMD including a rechargeable battery are desirable.